1. Field of the Invention
The present invention relates to a diaphragm-type sound-electricity conversion device produced by using a semiconductor microfabrication technique, and an array-type ultrasonic transducer and an ultrasonic diagnostic apparatus using the sound-electricity conversion device.
2. Description of the Related Art
In the Early Twentieth Century, experiments started for transmitting/receiving ultrasonic sound by using piezoelectric effect of quartz crystal, when a problem arose in that a crystal has low electromechanical conversion efficiency. This prevented obtaining a sufficient sensitivity for, in particular, a receiving transducer and thus achieving its application to a practical product. Then, Rochelle salt was discovered having high electromechanical conversion efficiency, which was used to develop a sonar during the Second World War. However, the Rochelle salt had a problem in crystal stability such as being very deliquescent, requiring a special attention to obtain stable piezoelectric characteristics.
After the War, barium titanate was discovered having high electromechanical conversion efficiency as well as stable piezoelectric effect. Being ceramic, barium titanate advantageously had a high degree of freedom in shape design, which led to creating the concept of “piezoelectric ceramic”. Then in the Late Twentieth Century was discovered lead zirconate titanate (PZT) ceramic having higher Curie point as well as even more stable piezoelectric effect than barium titanate. The emergence of PZT ceramic allowed obtaining a piezoelectric device with high sensitivity and stability. Thereafter, the piezoelectric device using PZT ceramic came to be widely used for, for example, ultrasonic transducers, as is found today.
The material replacement of the ultrasonic transducer from quartz crystal to the piezoelectric ceramic was advantageous in impedance matching in the accompanying replacement of electric circuits such as a receiving amplifier and a transmission drive circuit from vacuum tubes to semiconductors. However, the replacement of electric circuits including a drive circuit to semiconductors required meeting requirements in high voltage and high frequency operation, for example. Thus, it was necessary to wait for practical use of high-speed thyristors and highly resistant Field Effect Transistors (FETs). After the replacement to semiconductors was realized in electric circuits around the ultrasonic transducers, it was in the 1990s that studies started for forming a diaphragm-type ultrasonic transducer employing a semiconductor micro fabrication technique. The realization of such a semiconductor ultrasonic transducer using semiconductors allows forming an ultrasonic transducer and its peripheral circuits by a series of semiconductor fabrication processes, and therefore, notable effects can be expected in both production cost and performance of ultrasonic receivers.
A non-patent document by M. Haller and B. T. Khuri-Yakub, “A Surface Micromachined Electrostatic Ultrasonic Air Transducer”, Proceedings of Ultrasonic Symposium, pp. 1241-1244, 1 Nov. 1994, discloses an example of a sound-electricity conversion device in a diaphragm-type ultrasonic transducer produced with a semiconductor microfabrication technique. The sound-electricity conversion device has a basic structure in which an impurity-doped silicon substrate has on its top a cavity, a diaphragm of a silicon nitride film is formed opposite to the silicon substrate, the diaphragm and the substrate sandwiching the cavity, and further an electrode layer is formed on the surface or inside of the diaphragm on the cavity side.
That is, the basic structure of the sound-electricity conversion device was a capacitor having the silicon substrate as a lower electrode and the electrode layer formed on the diaphragm side as an upper electrode. Therefore, applying a voltage between these electrodes induces opposite electric charges on the electrodes, the charges attracting to each other and thereby displacing the diaphragm. At this time, if the diaphragm contacts on the outside with water or an organism, it radiates sound wave via the water or organism as a medium. Also, by applying a DC bias voltage on the electrode to induce thereon certain electric charge, and then forcibly applying vibration to the electrode from the medium contacting with the diaphragm, i.e., displacing the diaphragm, an additional voltage occurs between the electrodes depending on the displacement amount. This is the principle of sound-electricity conversion of the diaphragm-type ultrasonic transducer as shown in the above-cited non-patent document by M. Haller and B. T. Khuri-Yakub. The principle of sound-electricity conversion in ultrasonic reception is the same as the principle of a DC bias type capacitor microphone used as an audible sound range microphone.
The sound-electricity conversion device as discussed above has a diaphragm structure with a space on the back surface and therefore can obtain a good sound impedance matching to a mechanically soft material such as water and an organism, even if the device is configured with a mechanically hard material such as silicon. Also, because the sound-electricity conversion device is formed on the silicon substrate, it is possible to integrally form an ultrasonic transmission/reception circuit for driving the device on the same or closely arranged silicon substrate.
Thereafter, further studies and developments were made for the diaphragm-type ultrasonic transducer, which now has reached a level comparable with a piezoelectric type transducer using PZT in terms of, for example, transmission/reception sensitivity, although the basic structure and operation principle of the transducer have not greatly changed.
In a diaphragm-type ultrasonic transducer, in order to maximize its conversion efficiency, its electrodes are applied with a DC bias voltage of a magnitude that displaces the diaphragm close to contacting a silicon substrate so as to induce as much electric charge as possible. With this, the electrode on the diaphragm side easily contacts the silicon substrate. However, in practice, when the electrode on the diaphragm side contacts or come close to contacting the silicon substrate, a short-circuit occurs causing an excessive current flow or discharging phenomenon between the electrodes. In this occurrence, the excessive current, for example, may destroy the sound-electricity conversion device itself or the peripheral circuit system connected to the device.
Therefore, the current sound-electricity conversion device typically has a design in which at least one of the electrodes on the diaphragm and substrate sides is provided, on the cavity side, with an electrode short-circuit prevention film made from an insulation film. This electrode short-circuit prevention film can prevent a short-circuit or a discharge phenomenon from occurring between the electrodes, even when the electrode on the diaphragm side contacts the silicon substrate.
Such an electrode short-circuit prevention film is often formed of a silicon nitride film which is often formed by vapor phase epitaxy typified by CVD (Chemical Vapor Deposition). However, the silicon nitride film formed by CVD includes more coupling deficiencies than, for example, a silicon oxide film formed by thermal oxidation, and therefore is characteristically subject to electrification when applied with a high voltage. In addition, the amount of electric charge electrified drifts depending on the applied voltage value and with the passage of time, and does not stabilize.
That is, in a sound-electricity conversion device provided with an electrode short-circuit prevention film such as a CVD nitride film, such an unstable electric charge would occur between the capacitor electrodes indispensable to construct the principle of sound-electricity conversion. Therefore, even if the same voltage is applied between the electrodes or if the diaphragm electrodes are displaced by the same amount, the amount of electric charge induced in the electrodes would change and drift. This causes the sound-electricity conversion characteristics of the sound-electricity conversion device to drift and become unstable.
The drift of the sound-electricity conversion characteristics has a critical effect on the characteristics of an array-type ultrasonic transducer constructed by arranging many of such sound-electricity conversion devices. This is because when the sound-electricity conversion characteristics of each of the devices constructing the array-type ultrasonic transducer drift independently, the entirety of an ultrasonic diagnostic apparatus using the array-type ultrasonic transducer experiences a considerable increase in the sound noise level when forming transmission and reception beams. As discussed above, the ultrasonic transducer using the semiconductor diaphragm type sound-electricity conversion device has not sufficiently solved the problems of sensitivity and stability.
In view of the above-mentioned problems in the prior art, the present invention aims to stabilize the sound-electricity conversion characteristics of the sound-electricity conversion device provided with the electrode short-circuit prevention film, and to decrease the sound noise level of the ultrasonic transducer as well as the ultrasonic diagnostic apparatus configured by the sound-electricity conversion device.